Insoluble electrodes are commonly employed in processes for coating strips of metals by electrochemical routes, preferably of zinc-coated or galvanized steel strips, with the aid of metals or metal alloys, in accordance with which an electrolyte laden with salts of the coating metals is recycled between the cathodic metal strip to be coated and the insoluble anode.
Depending on the electrolyte used, for example sulphate- or chloride-based electrolytes, the use of this process generates gases at the anode, for example oxygen or chlorine, which partially undergo undesirable bonding with the coating metals or which, because of their high reactivity or their toxicity, have detrimental consequences during the use of the coating process or for the environment. These gases which arise at the anode mix with the electrolyte and can, consequently, cause undesired reactions and enter the environment, given that during the electrolytic coating of metal strips the electrolyte circuit on the strip cannot be separated from the atmosphere.
A process for coating galvanized steel strips with the aid of iron compounds or of alloys of the latter is known. To do this, a sulphate-based electrolyte laden with salts of coating metal is conducted in a closed circuit between the steel strip to be coated circulating endlessly and the insoluble anodes. Because of known electrochemical processes, iron precipitates in the form of iron compounds at the cathodic metal strip. Divalent oxygen is released at the anode, oxygen which comes into contact with the metal salts, more particularly because of the recycling of the electrolyte. This oxygen oxidizes a part of the divalent iron to trivalent iron, with the result that large quantities of iron oxide are produced, which soil the electrolytes and which must be separated from the circuit by making use of costly filtration processes.
Furthermore, the formation of Fe.sup.3+ reduces the cathodic efficiency of the current and deteriorates the adhesion of the deposited layer.
Finally, the use of salts of the coating metal, or the use of iron in corresponding dissolving plants and the replacement of other substances employed which are entrained together increase the cost of such a coating process very appreciably.